Making a warlock BBEG interesting?

[Super Evil User]

So in one of our old campaigns, we had a recurring villain early on who was a level 8 warlock. Her motivation was that she needed an amulet to summon her balor master into the world and her personality was basically Morrigan from Dragon Age. We even called her "Morrigan" for shorthand (her real name was Hekaerge and that was hard to remember) when we were in a hurry.

We've decided to use her again, but this time for an E6 campaign. Since this is an E6 campaign, this gives me the opportunity to totally beef up her backstory.

I'm thinking that maybe she got that powerful for a reason. Maybe she was the result of a magical experiment gone awry, and she needs the amulet to stabilize her rapidly decaying condition?

[Kaeso]

So in one of our old campaigns, we had a recurring villain early on who was a level 8 warlock. Her motivation was that she needed an amulet to summon her balor master into the world and her personality was basically Morrigan from Dragon Age. We even called her "Morrigan" for shorthand (her real name was Hekaerge and that was hard to remember) when we were in a hurry.

We've decided to use her again, but this time for an E6 campaign. Since this is an E6 campaign, this gives me the opportunity to totally beef up her backstory.

I'm thinking that maybe she got that powerful for a reason. Maybe she was the result of a magical experiment gone awry, and she needs the amulet to stabilize her rapidly decaying condition?

Cool. I'm actually going for a sort of similar thing in my own campaign (which is E6 as well).
As for your idea, that sounds very plausible. Think about it, she's the agent of some demonic force. This demonic force would imbue her with amazing powers far beyond what a normal human can ever reach (ie. beyond the E6 limit), but at a great cost. It would make sense that, as time passes, her health would deteriorate and she would eventually die a horrible death. The only way to prevent this would be to actually summon her Balor master and his demonic host into this world, so he can lift her curse and reign havoc over the world.

After all, a Balor is not benevolent. He could give amazing powers to a human, but only when he expects something in return. It's a Faustian deal.

[Super Evil User]

Oh no, the level cap's at 10 for NPCs. The E6 thing's strictly a PC restriction only.

Level 10 folks are usually archmages or Genghis Khan-style warlords, though...

[Kymme]

Hm... have you heard of Chameleon? It's a prestige class from Races of Destiny that synergies pretty well with warlock. Chameleons can change their focus every day, gaining new class features and in some cases, spell lists. Also, at level 2 a Chameleon gains a floating feat that they can swap out at the start of each day. I've heard of that floating feat being used to take Extra Invocation to gain a new power to play around with every day.

You could check that out. If you still want her to be level 8, you could go Warlock 6/Chameleon 2. That way she can use her floating feat to take Lesser Invocations.